An artist's interpretation shows a cloudy exoplanet with its home star in the background. Data from the Hubble Space Telescope show that warm Neptune GJ 436b and super-Earth GJ 1214b may have high-altitude clouds covering their atmospheres.

An artist's interpretation shows a cloudy exoplanet with its home star in the background. Data from the Hubble Space Telescope show that warm Neptune GJ 436b and super-Earth GJ 1214b may have high-altitude clouds covering their atmospheres. (Space Telescope Science Institute)

Amina Khan

Using NASA’s Hubble Space Telescope, two research teams have discovered thick layers of high-altitude clouds covering the atmospheres of two relatively nearby exoplanets: a super-Earth and a "warm Neptune."

Scientists are beginning to get a handle on how to study the atmospheres on alien planets, and have even found green clouds on the super-Jupiter planet, Kepler-7b. But the findings, published in the journal Nature this week, show that clouds can cover smaller planets too — including two of the most common types of planets found in the Milky Way.

Neither of the planets, GJ 436b and GJ 1214b, are quite like anything found in our solar system. GJ 436b is a warm Neptune that’s slightly larger than Neptune and much closer to its host star. Thus unlike our own distant, gassy ice giant, GJ 436b’s surface temperature sits about a blistering 980 degrees Fahrenheit. It is 36 light-years from Earth in the constellation Leo. GJ 1214b, a super-Earth whose radius is 2.7 times that of our home planet, sits about 40 light-years away in the constellation Ophiuchus.

"Super-Earth and Neptune-class planets collectively represent an intriguing and populous type of extrasolar planet whose exotic atmosphere may have no true analogies in the solar system," Julianne Moses of the Space Science Institute in Boulder, Colo., who was not involved in the research, wrote in a commentary on the papers.

To look at their atmospheres, the researchers wait for the planet to cross in front of its star and look at the starlight filtering through the backlit planet’s thin layer of atmosphere. They usually look for dips in certain wavelengths of light — a luminous fingerprint caused by chemicals in the atmosphere, which would reveal what the planet’s shell of air was made of.

But for these planets, they found no such chemical fingerprint imprinted on the light. The most likely reason, the researchers surmised, would be a layer of high-altitude clouds blocking the view.

"However, high-altitude clouds on these two exoplanets would not resemble the clouds we see in the solar system," Moses wrote. "Possible candidates include potassium chloride or zinc sulphide 'dust' clouds."